Tools that enable picking and positioning of individual atoms and molecules for chemical reaction could have widespread applications in chemical synthesis and materials science (e.g. molecular nanotechnology). Such tools, or assemblers, could combine the chemical diversity of synthetic organic chemistry (e.g. functional groups), which is chiefly realized at the atomic level, with the ability of human engineers to fabricate objects using mechanical devices (e.g. robotic welders), which is chiefly realized at the macroscopic level, for the development of a universal molecular manufacturing scheme. See K. E. Drexler, Proc. Natl. Acad. Sci. USA 78, 5275 (1981); K. E. Drexler, Sci. Amer. 285, 74 (2001); G. M. Whitesides, Sci. Amer. 285, 78 (2001); K. C. Nicolau, D. Vourloumis, N. Winssinger, P. S. Baran, Angew. Chem. Int. Ed. Engl. 39, 44 (2000); S. Hecht, Angew. Chem. Int. Ed. Engl. 42, 24 (2003); and K. E. Drexler, Nanosystems. Molecular Machinery, Manufacturing and Computation; Wiley-Interscience: New York (1992).
Owing to the superior stereo- and regiocontrol of chemical synthesis offered by assemblers, molecular manufacturing via molecular assemblers could provide efficient, low-cost access to molecules, and materials, with unique molecular (e.g. catalytic) and bulk physical (e.g. mechanical) properties. Despite this realization, however, a synthetic system that enables such general ‘pick-and-place’ control of atoms and molecules has not been realized.
Top-down approaches, which have employed surface microscopy tips (e.g. STM) to position reactive sites mechanically, have had difficulties achieving atomic-level dexterity and high throughput for grabbing individual molecules and manufacturing appreciable amounts of product, respectively. See S. Hecht, Angew. Chem. Int. Ed. Engl. 42, 24 (2003); V. Balzani, A. Credi, M. Venturi, Chem. Eur. J. 8, 5525 (2002); D. M. Eigler and E. K. Schweizer, Nature 344, 594 (1990); and S.-W. Hla, G. Meyer, K.-H. Rieder, ChemPhysChem. 2, 361 (2001).
Bottom-up approaches, which have employed molecules to recognize and assemble reactive sites chemically, have had difficulties contending with structure effects of entropy and solvent of the liquid phase, which can hinder reactants molecules from achieving the necessary order for reaction. See V. Balzani, A. Credi, M. Venturi, Chem. Eur. J. 8, 5525 (2002); J.-M. Lehn, Supramolecular Chemistry; Wiley-VCH: Weinheim (1995); T. R. Kelly, C. Zhao, G. J. Bridger, J. Am. Chem. Soc. 111, 3744 (1989); and D. M. Bassani, V. Darcos, S. Mahony, J.-P. Desvergne, J. Am. Chem. Soc. 122, 8795 (2000).
A [n]-ladderane is a molecule that consists of n edge-sharing cyclobutane rings (where n≧2) that define a molecular equivalent of a macroscopic ladder. See H. Hopf, Angew. Chem. 2003, 115, 2928-2931; and Angew. Chem., Int. Ed. 2003, 42, 2822-2825. Ladderanes are considered promising building blocks in optoelectronics and, very recently, have been identified in biological systems (where: n=3 and 5), in the form of ladderane lipids, being integral components in the microbiological conversion of ammonium and nitrite to dinitrogen gas. See W. Li, M.A. Fox, J. Am. Chem. Soc. 1996, 118, 11752-11758; J.S. S. Damste, et al., Nature 2002, 419, 708-712; E. F. DeLong Nature 2002, 419, 676-677; and M. M. M. Kuypers, et al., Nature 2003, 422, 608-611.
In the simplest case, a cis-fused [n]-ladderane (n=3, 5, 7 . . . ) can be constructed by photochemical dimerization of two all-trans-poly-m-enes (m=2, 3, 4.). Despite the apparent simplicity of this intermolecular process, however, such a transformation generally fails. This can be attributed to the lack of a method that overcomes the energetic cost, due to solvent and entropy effects, of organizing two polyene molecules in a suitable geometry in the liquid phase for photoreaction, although a covalent linker that holds two polyene chains in a parallel orientation for a high-yield, intramolecular photoaddition to give a [n]-ladderane (where: n=3 and 5) has been reported. See H. Hopf, Angew. Chem. 2003, 115, 2928-2931; and Angew. Chem., Int. Ed. 2003, 42, 2822-2825; D. H. Williams, E. et al., Chem. Commun. 2003, 1973-1976; M. Rekharsky, et al., J. Am. Chem. Soc. 2002, 124, 14959-14967; and H. Hopf, et al., Angew. Chem. 1995, 107, 742-744; Angew. Int. Ed. Engl. 1995, 34, 685-687.
Unfortunately, the study of these unique molecules and their properties has been hampered because existing methods for preparing ladderanes typically provide low yields and/or mixtures of products that are difficult to separate. Accordingly, there is currently a need for improved methods and intermediates that can be used to prepare ladderanes. In particular, there is a need for methods that provide improved yields of ladderanes and for methods that provide pure ladderane products as opposed to mixtures of compounds.